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234
years ; the diseases' referred to being typhoid fever and cholera.
At the present moment, when cholera has reappeared on the
continent, it is reassuring to know that " recent bacteriological
research .... has demonstrated that sand filtration is practically
a perfect safeguard against the introduction of the cholera
bacillus into drinking water." The bacterioscopic examination
of the water supply has become, therefore, " an
investigation of the highest importance from a hygienic
point of vew," and to this Dr. Frankland is "each
year devoting an increased amount of attention." The
samples of waters examined are obtained immediately
after the water has left the filters: thus the " maximum
degree of sterility of each water is determined"—
after all sources of contamination have been passed. The
fewer " microbes found in a given volume at that moment, the
less is the probability of pathogenic organisms being present."
In the initial absence of such organisms, the indefinite multiplication
of non-pathogenic organisms is of no consequence.
In illustration of the bacterial condition of the Thames at the
intakes, samples of raw river water, collected at the same time
as the filtered water, are submitted to examination, and also
samples pumped from the natural gravel beds near the river.
The " bacterial amelioration " of this latter water is remarkable
: " no such high degree of purification has been as yet
obtained by gravel laid artificially for this purpose." It is
shown that a "low temperature favours, probably not the
multiplication but, at all events, the preservation of living
spores in the river water;" the number found in the colder
months being far in excess of those met with in the warmer
months of the year. A great destruction of bacteria, moreover,
is effected by storage. Particulars in regard to this are given
in relation to the water supplied by the Chelsea Company,
which has storage capacity equal to 14 7 days' supply, and
filters the water through four feet of sand at the average rate
of 1¾ gallons per square foot per hour. The average percentage